Role of PV-Powered Vehicles in Low-Carbon Society and Some Approaches of High-Efficiency Solar Cell Modules for Cars

Development of highly-efficient photovoltaic (PV) modules and expanding its application fields are significant for the further development of PV technologies and realization of innovative green energy infrastructure based on PV. Especially, development of solar-powered vehicles as a new application is highly desired and very important for this end. This paper presents the impact of PV cell/module conversion efficiency on reduction in CO2 emission and increase in driving range of the electric based vehicles. Our studies show that the utilization of a highly-efficient (higher than 30%) PV module enables the solar-powered vehicle to drive 30 km/day without charging in the case of light weight cars with electric mileage of 17 km/kWh under solar irradiation of 3.7 kWh/m2/day, which means that the majority of the family cars in Japan can run only by the sunlight without supplying fossil fuels. Thus, it is essential to develop high-efficiency as well as low-cost solar cells and modules for automotive applications. The analytical results developed by the authors for conversion efficiency potential of various solar cells for choosing candidates of the PV modules for automotive applications are shown. Then we overview the conversion efficiency potential and recent progress of various Si tandem solar cells, such as III-V/Si, II-VI/Si, chalcopyrite/Si, and perovskite/Si tandem solar cells. The III-V/Si tandem solar cells are expected to have a high potential for various applications because of its high conversion efficiency of larger than 36% for dual-junction and 42% for triple-junction solar cells under 1-sun AM1.5 G illumination, lightweight and low-cost potentials. The analysis shows that III-V based multi-junction and Si based tandem solar cells are considered to be promising candidates for the automotive application. Finally, we report recent results for our 28.2% efficiency and Sharp’s 33% mechanically stacked InGaP/GaAs/Si triple-junction solar cell. In addition, new approaches which are suitable for automotive applications by using III-V triple-junction, and static low concentrator PV modules are also presented.

Organolead halide perovskite：A rising player in high-efficiency solar cells

这个观点介绍对 organolead 卤化物的简短描述基于 perovskite 的太阳能电池包括结构和 perovskite 的基本性质，太阳能电池的分类，和他们是的潜力的眼界双人脚踏车的 subcells 光电的设备和大规模适用性。

Hexagonal boron phosphide and boron arsenide van der Waals heterostructure as high-efficiency solar cell

The rapid development of two-dimensional(2D)materials offers new opportunities for 2D ultra-thin excitonic solar cells(XSCs).The construction of van der Waals heterostructure(vdWH)is a recognised and effective method of integrating the properties of single-layer 2D materials,creating particularly superior performance.Here,the prospects of h-BP/h-BAs vdW heterostructures in 2D excitonic solar cells are assessed.We systematically investigate the electronic properties and optical properties of heterogeneous structures by using the density functional theory(DFT)and first-principles calculations.The results indicate that the heterogeneous structure has good optoelectronic properties,such as a suitable direct bandgap and excellent optical absorption properties.The calculation of the phonon spectrum also confirms the well-defined kinetic stability of the heterstructure.We design the heterogeneous structure as a model for solar cells,and calculate its solar cell power conversion efficiency which reaches up to 16.51%and is higher than the highest efficiency reported in organic solar cells(11.7%).Our work illustrates the potential of h-BP/h-BAs heterostructure as a candidate for high-efficiency 2D excitonic solar cells.

Accelerated Sequential Deposition Reaction via Crystal Orientation Engineering for Low-Temperature,High-Efficiency Carbon-Electrode CsPbBr_(3) Solar Cells

Low-temperature,ambient processing of high-quality CsPbBr_(3)films is demanded for scalable production of efficient,low-cost carbon-electrode perovskite solar cells(PSCs).Herein,we demonstrate a crystal orientation engineering strategy of PbBr_(2)precursor film to accelerate its reaction with CsBr precursor during two-step sequential deposition of CsPbBr_(3)films.Such a novel strategy is proceeded by adding CsBr species into PbBr_(2)precursor,which can tailor the preferred crystal orientation of PbBr_(2)film from[020]into[031],with CsBr additive staying in the film as CsPb_(2)Br_(5)phase.Theoretical calculations show that the reaction energy barrier of(031)planes of PbBr_(2)with CsBr is lower about 2.28 eV than that of(O2O)planes.Therefore,CsPbBr_(3)films with full coverage,high purity,high crystallinity,micro-sized grains can be obtained at a low temperature of 150℃.Carbon-electrode PSCs with these desired CsPbBr_(3)films yield the record-high efficiency of 10.27%coupled with excellent operation stability.Meanwhile,the 1 cm^(2)area one with the superior efficiency of 8.00%as well as the flexible one with the champion efficiency of 8.27%and excellent mechanical bending characteristics are also achieved.

An additive dripping technique using diphenyl ether for tuning perovskite crystallization for high-efficiency solar cells

Controlling the morphology of the MAPbI3-xClx active layer has remained a challenge towards advancing perovskite solar cells （PvSCs）. Here, we demonstrate that a low temperature additive dripping （AD） treatment step, using diphenyl ether （DPE）, can significantly improve the power conversion efficiency （PCE）, compared to the control device using chlorobenzene （CB）, by 15% up to 16.64%, with a high current density （Jsc） of 22.67 mA/cm^2. We chose DPE for its small and appropriate dipole moment to adjust the solubility of the MAPbI3-xClx precursor during the formation of the intermediate phase and the MAPbI3-xClx phase. The low DPE vapor pressure provides a longer processing window for the removal of residual dimethylformamide （DMF）, during the annealing process, for improved perovskite formation. Imaging and X-ray analysis both reveal that the MAPbI3-xClx film exhibits enlarged grains with increased crystallinity. Together, these improvements result in reduced carrier recombination and hole trap-state density in the MAPbI3-xClx film, while minimizing the hysteresis problem typical of PvSCs. These results show that the AD approach is a promising technique for improving PvSCs.

Benzothiadiazole-based hole transport materials for high-efficiency dopant-free perovskite solar cells: Molecular planarity effect

A new benzothiadiazole-based D-A-D hole transport material(DTBT)has been designed and synthesized with a more planar structure by introducing of thiophene bridges.The results indicate a lower band gap and quite higher hole mobility for the DTBT.Furthermore,the enhancement in molecular planarity with simple thiophene unit increases the hole mobility of DTBT(8.77×10^-4cm^2 V^-1s^-1)by about 40%.And when DTBT is used as hole transport material in perovskite solar cells,the photoelectric conversion efficiency of the corresponding dopant-free devices is also significantly improved compared with that of the conventional BT model molecule without thiophene.In terms of device stability,DTBT-based devices show a favorable long-term stability,which keep 83%initial efficiency after 15 days.Therefore,the introducing of thiophene bridges in D-A-D typed HTMs can improve the molecular planarity effectively,thereby increasing the hole mobility and improving device performance.

Moth-eye Structured Polydimethylsiloxane Films for High-Efficiency Perovskite Solar Cells

Large-area polydimethylsiloxane(PDMS)films with variably sized moth-eye structures were fabricated to improve the efficiency of perovskite solar cells.An approach that incorporated photolithography,bilayer PDMS deposition and replication was used in the fabrication process.By simply attaching the moth-eye PDMS films to the transparent substrates of perovskite solar cells,the optical properties of the devices could be tuned by changing the size of the moth-eye structures.The device with 300-nm moth-eye PDMS films greatly enhanced power conversion efficiency of ~21 % due to the antireflective effect of the moth-eye structure.Furthermore,beautiful coloration was observed on the 1000-nm moth-eye PDMS films through optical interference caused by the diffraction grating effect.Our results imply that moth-eye PDMS films can greatly enhance the efficiency of perovskite solar cells and building-integrated photovoltaics.

Well-aligned Ni Pt alloy counter electrodes for high-efficiency dye-sensitized solar cell applications

Development of cost-effective and robust counter electrodes(CEs) is a persistent objective for highefficiency dye-sensitized solar cells(DSSCs). To achieve this goal, we present here the hydrothermal synthesis of well-aligned Ni Pt alloy CEs, which is templated by ZnO nanowires and nanosheets. The preliminary results demonstrate that Ni Pt alloy electrodes are featured by increased charge-transfer processes and electrocatalytic activity in comparison with expensive Pt CE, yielding power conversion efficiencies of 8.29% and 7.41% in corresponding DSSCs with Ni Pt nanowire and nanosheet alloy CEs, respectively. Additionally, the Ni Pt alloy CEs also display extraordinary dissolution-resistant ability when suffering longterm utilization in liquid-junction DSSCs.

Facile preparation of organometallic perovskite films and high-efficiency solar cells using solid-state chemistry

The power conversion efficiency of organometallic perovskite-based solar cells has skyrocketed in recent years. Intensive efforts have been made to prepare high-quality perovskite films tailored to various device configurations. Planar heterojunction devices have achieved record efficiencies; however, the preparation of perovskite films for planar junction devices requires the use of expensive vacuum facilities and/or the fine control of experimental conditions. Here, we demonstrate a facile preparation of perovskite films using solid-state chemistry. Solid-state precursor thin films of CHBNH3I and PbI2 are brought into contact with each other and allowed to react via thermally accelerated diffusion. The resulting perovskite film displays good optical absorption and a smooth morphology. Solar cells based on these films show an average efficiency of 8.7% and a maximum efficiency of 10%. The solid-state synthesis of organometallic perovskite can also be carried out on flexible plastic substrates. Using this method on a PET/ITO substrate produces devices with an efficiency of 3.2%. Unlike existing synthetic methods for organometallic perovskite films, the solid-state reaction method does not require the use of orthogonal solvents or careful adjustment of reaction conditions, and thus shows good potential for mass production in the future.

High-efficiency ultra-thin Cu_(2)ZnSnS_(4) solar cells by double-pressure sputtering with spark plasma sintered quaternary target

In recent years,Cu_(2)ZnSnS_(4)(CZTS)semiconductor materials have received intensive attention in the field of thin-film solar cells owing to its non-toxic and low-cost elements.In this work,double-pressure sputtering technology is applied to obtain highly efficient and ultra-thin(-450 nm)pure Cu_(2)ZnSnS_(4)(CZTS)solar cell.Using mixed materials with sulfides and copper powder as a quaternary target via spark plasma sintering(SPS)method and adopting double-layer sputtering(high+low pressure),a highly adhesive and large-grained CZTS thin film is achieved.As a result,the damage to the surface of Mo contact is decreased so that the reflectivity of incident light can be improved.Moreover,the composition of CZTS film was more uniform and the secondary phase separation at the Mo interface was reduced.Therefore,the interface defect state and deep level defect density in corresponding device with double-pressure is reduced and the ratio of depletion thickness to absorption layer thickness can reached to 0.58,which promoted the collection of photogenerated carriers.Finally,an efficiency of 9.3%for ultra-thin(~450 nm)CZTS film solar cell is obtained.

Toward high-efficiency perovskite solar cells with one-dimensional oriented nanostructured electron transport materials

The unique advantages of one-dimensional(1D)oriented nanostructures in light-trapping and chargetransport make them competitive candidates in photovoltaic(PV)devices.Since the emergence of perovskite solar cells(PSCs),1D nanostructured electron transport materials(ETMs)have drawn tremendous interest.However,the power conversion efficiencies(PCEs)of these devices have always significantly lagged behind their mesoscopic and planar counterparts.High-efficiency PSCs with 1D ETMs showing efficiency over 22%were just realized in the most recent studies.It yet lacks a comprehensive review covering the development of 1D ETMs and their application in PSCs.We hence timely summarize the advances in 1D ETMs-based solar cells,emphasizing on the fundamental and optimization issues of charge separation and collection ability,and their influence on PV performance.After sketching the classification and requirements for high-efficiency 1D nanostructured solar cells,we highlight the applicability of 1D TiO_(2)nanostructures in PSCs,including nanotubes,nanorods,nanocones,and nanopyramids,and carefully analyze how the electrostatic field affects cell performance.Other kinds of oriented nanostructures,e.g.,ZnO and SnO_(2)ETMs,are also described.Finally,we discuss the challenges and propose some potential strategies to further boost device performance.This review provides a broad range of valuable work in this fast-developing field,which we hope will stimulate research enthusiasm to push PSCs to an unprecedented level.

Interfaces of high-efficiency kesterite Cu_2ZnSnS(e)_4 thin film solar cells

Cu2ZnSnS（e）4 （CZTS（e）） solar cells have attracted much attention due to the elemental abundance and the non- toxicity. However, the record efficiency of 12.6% for CuzZnSn（S,Se）4 （CZTSSe） solar cells is much lower than that of Cu（In,Ga）See （CIGS） solar cells. One crucial reason is the recombination at interfaces. In recent years, large amount inves- tigations have been done to analyze the interfacial problems and improve the interfacial properties via a variety of methods. This paper gives a review of progresses on interfaces of CZTS（e） solar cells, including： （i） the band alignment optimization at buffer/CZTS（e） interface, （ii） tailoring the thickness of MoS（e）2 interfacial layers between CZTS（e） absorber and Mo back contact, （iii） the passivation of rear interface, （iv） the passivation of front interface, and （v） the etching of secondary phases.

A facile solution processed ZnO@ZnS core–shell nanorods arrays for high-efficiency perovskite solar cells with boosted stability

Zinc Oxide(ZnO)has been extensively applied as electron transport material(ETM)in perovskite solar cells(PSCs)since the emergence of PSCs.However,some chemisorbed oxygen species on the surface of ZnO can cause the degradation of CH3NH3+(MA^(+))based perovskite.To avoid the destructive effect of ZnO,a facile solution strategy was proposed to produce a ZnS shell around the ZnO nanorods arrays(ZnO-NRs),i.e.ZnO@ZnS core-shell nanorods(ZnO-NRs@ZnS).The ZnO-NRs@ZnS cascade structure can not only facilitate carrier transport,but also enhance the stability of ZnO based PSCs.A power conversion efficiency(PCE)of 20.6%was finally yielded,which is the-state-of-the-art efficiency for PSCs with one-dimensional(1 D)ZnO electron transport materials(ETMs).Moreover,over 90%of the initial efficiency was retained for the unencapsulated device with ZnO-NRs@ZnS ETMs at 85℃for 500 h,demonstrating excellent stability.This work provides a simple and efficient avenue to simultaneously enhance the photovoltaic(PV)performance and stability of 1 D ZnO nanostructure-based PSCs.

Approaches for High-Efficiency III-V/Si Tandem Solar Cells

The Si tandem solar cells are very attractive for realizing high efficiency and low cost. This paper overviews current status of III-V/Si tandem solar cells including our results. The analytical results for efficiency potential of Si tandem solar cells and loss analysis of Si bottom cells as well as bandgap energy optimization of sub-cells are presented. The 2-junction and 3-junction Si tandem solar cells have potential efficiencies of 36% and 42%, respectively. ERE (external radiative efficiency) analysis for Si solar cells is analyzed in order to clarify properties of Si bottom solar cells. Properties of single-crystalline Si heterojunction solar cell fabricated in this study were analyzed. The current status of efficiencies of our Si bottom cell, upper III-V 2-junction solar cell and III-V/Si 3-junction tandem solar cell was shown to be 5.2% and 28.6% and 33.8%. Achievement of Jsc of 12 mA/cm2 for Si bottom cell is necessary to realize high-efficiency 3-junction Si tandem solar cells with an efficiency of more than 37%. In addition, this paper presents ERE analysis of III-V 2-junction upper solar cells for improving III-V/Si 3-junction tandem solar cells. Several ways to improve efficiency of III-V/Si 3-junction tandem solar cells by reducing non-radiative recombination, optical and resistance losses are shown.

Loss Analysis of High-Efficiency Perovskite/Si Tandem Solar Cells for Large Market Applications

The Si tandem solar cells composes of III-V, II-VI, chalcogenide and perovskite top cells and Si bottom cells are very attractive for creation of new markets. The perovskite/Si tandem solar cells are thought to be one of the most promising PV devices because of high-efficiency and low-cost potential. However, efficiencies of perovskite/Si tandem solar cells with an efficiency of 29.8% are lower compared to 39.5% with III-V 3-junction tandem solar cells and 35.9% with III-V/Si 3-junction tandem solar cells. Therefore, it is necessary to clarify and reduce several losses of perovskite/Si tandem solar cells. This paper presents high efficiency potential of perovskite/Si tandem solar cells analyzed by using our analytical procedure and discusses about non-radiative recombination, optical and resistance losses in those tandem solar cells. The perovskite/Si 2-junction tandem solar cells is shown to have efficiency potential of 37.4% as a result of non-radiative recombination loss of 2.3%, optical loss of 2.7% and resistance loss of 3.1%. Although the perovskite/Si 3-junction tandem solar cells are thought to be very attractive because of higher efficiency with an efficiency of more than 42%, decreasing non-radiative recombination loss in wide bandgap perovskite solar cell materials is pointed out to be necessary.

In-situ photoisomerization of azobenzene to inhibit ion-migration for stable high-efficiency perovskite solar cells

Ion migration is a notorious problem in perovskite solar cells(PSCs)that severely mutilates device performance.Herein,a strategy to inhibit ion migration in situ is developed by using photoisomerization of azobenzene(AZO)to immobilize cations in the lattice.During the nucleation process,the photoisomerized cis-AZO reacts with FA^(+),MA^(+)and Pb2+cations in the perovskite precursor by synergistic cation-πinteraction and Lewis base coordination,leading to heterogeneous nucleation to produce uniform perovskite film.Meanwhile,it accelerates conversion of intermediate yellowδ-phase to desired black aphase of FAPb I3for improved crystallinity with well-passivated grain surface.Consequently,defect density is effectively reduced for the perovskite film to demonstrate suppressed carrier recombination and enhanced carrier extraction.Subsequently,the solar cell efficiency is elevated from 21.29%to 23.58%with negligible J-V hysteresis.Long-term stability is also improved,with the bare device without any encapsulation retaining 84%of its initial efficiency after aging 744 hours in ambient.

Surface-Modified Graphene Oxide/Lead Sulfide Hybrid Film-Forming Ink for High-Efficiency Bulk Nano-Heterojunction Colloidal Quantum Dot Solar Cells

Solution-processed colloidal quantum dot solar cells(CQDSCs) is a promising candidate for new generation solar cells.To obtain stable and high performance lead sulfide(PbS)-based CQDSCs,high carrier mobility and low non-radiative recombination center density in the PbS CQDs active layer are required.In order to effectively improve the carrier mobility in PbS CQDs layer of CQDSCs,butylamine(BTA)-modified graphene oxide(BTA@GO) is first utilized in PbS-PbX2(X=I-,Br-) CQDs ink to deposit the active layer of CQDSCs through one-step spin-coating method.Such surface treatment of GO dramatically upholds the intrinsic superior hole transfer peculiarity of GO and attenuates the hydrophilicity of GO in order to allow for its good dispersibility in ink solvent.The introduction of B TA@GO in CQDs layer can build up a bulk nano-heterojunction architecture,which provides a smooth charge carrier transport channel in turn improves the carrier mobility and conductivity,extends the carriers lifetime and reduces the trap density of PbS-PbX2 CQDs film.Finally,the BTA@GO/PbS-PbX2 hybrid CQDs film-based relatively large-area(0.35 cm2) CQDSCs shows a champion power conversion efficiency of 11.7% which is increased by 23.1% compared with the control device.

Review on Metallization Approaches for High-Efficiency Silicon Heterojunction Solar Cells

Crystalline silicon(c-Si)heterojunction(HJT)solar cells are one of the promising technologies for next-generation industrial high-efficiency silicon solar cells,and many efforts in transferring this technology to high-volume manufacturing in the photovoltaic(PV)industry are currently ongoing.Metallization is of vital importance to the PV performance and long-term reliability of HJT solar cells.In this review,we summarize the development status of metallization approaches for highefficiency HJT solar cells.For conventional screen printing technology,to avoid the degradation of the passivation properties of the amorphous silicon layer,a low-temperature-cured(<250℃)paste and process are needed.This process,in turn,leads to high line/contact resistances and high paste costs.To improve the conductivity of electrodes and reduce the metallization cost,multi-busbar,fine-line printing,and low-temperature-cured silver-coated copper pastes have been developed.In addition,several potential metallization technologies for HJT solar cells,such as the Smart Wire Contacting Technology,pattern transfer printing,inkjet/FlexTrailprinting,and copper electroplating,are discussed in detail.B ased on the summary,the potential and challenges of these metallization technologies for HJT solar cells are analyzed.

A bilateral cyano molecule serving as an effective additive enables high-efficiency and stable perovskite solar cells

The existence of defects in perovskite films is a major obstacle that prevents perovskite solar cells (PSCs) from high efficiency and long-term stability. A variety of additives have been introduced into perovskite films for reducing the number of defects. Lewis base-based additive engineering has been considered as an effective way to eliminate defects, especially the defects caused by the uncoordinated Pb^(2+). In this work, for the first time, a bilateral cyano molecule (succinonitrile, SN) which is a commonly used plasticizer in solid electrolyte of solid-state lithium batteries was selected as an additive to modify organic–inorganic hybrid perovskite films in PSCs. SN is featured with two cyano groups (–C≡N) distributing at both terminals of the carbon chain, providing two cross-linking points to interact with perovskites crystals via coordinating with uncoordinated Pb2+ and forming hydrogen bonds with –NH2 groups in perovskite. It was found that the addition of SN into perovskite precursor solution could effectively reduce defects, particularly inhibit the appearance of Pb0 and thus suppress trap-assisted nonradiative charge carrier recombination. As a result, the efficiency of CH_(3)NH_(3)PbI_(3)(Cl) (MAPbI_(3)(Cl))-based PSCs was improved from 18.4% to 20._(3)% with enhanced long-term stability at N2 and humid air atmosphere. This work provides a facile and effective strategy to enhance the PCE and stability of PSCs simultaneously, facilitating the commercialization of PSCs.

Control of aggregated structure of photovoltaic polymers for high-efficiency solar cells

π-Conjugated organic/polymer materials-based solar cells have attracted tremendous research interest in the fields of chemistry,physics,materials science,and energy science.To date,the best-performance polymer solar cells(PSCs)have achieved power conversion efficiencies exceeding 18%,mostly driven by the molecular design and device structure optimization of the photovoltaic materials.This review article provides a comprehensive overview of the key advances and current status in aggregated structure research of PSCs.Here,we start by providing a brief tutorial on the aggregated structure of photovoltaic polymers.The characteristic parameters at different length scales and the associated characterization techniques are overviewed.Subsequently,a variety of effective strategies to control the aggregated structure of photovoltaic polymers are discussed for polymer:fullerene solar cells and polymer:nonfullerene small molecule solar cells.Particularly,the control strategies for achieving record efficiencies in each type of PSCs are highlighted.More importantly,the in-depth structure-performance relationships are demonstrated with selected examples.Finally,future challenges and research prospects on understanding and optimizing the aggregated structure of photovoltaic polymers and their blends are provided.